1. Field of the Invention
The present invention relates to a surface acoustic wave filter using a ladder type band pass filter that uses a surface acoustic wave resonator in small mobile communication equipment such as portable telephones, specifically to a surface acoustic wave filter that can be used even with a high power applied.
2. Description of the Background Art
There have been keen and energetic developments of small and lightweight mobile communication terminals such as portable telephones. The developments have involved demands for smaller parts and higher performances of the components used. These demands result in developments of RF (radio frequency) components using the surface acoustic wave (SAW) filter, which are used in practice. The SAW filter is a device that makes a great contribution to miniaturization of the RF units, which has been actively developed and used partly in some areas.
The recent improvements of performances in the mobile communication terminals have led to a demand for a high performance SAW branching filter that has a still lower insertion loss in the pass band and a higher attenuation in the attenuation band. The SAW branching filter is composed of a Tx filter (transmitting filter), Rx filter (receiving filter), and branching line, so that the transmission signal and reception signal may not interfere in branching and generating signals in the mobile communication terminals.
Especially, in the SAW branching filter for mobile communication terminals, the SAW filter is preferred not to have deterioration of the characteristics even when a high power is applied thereto. The SAW filter that can handle a high power is disclosed, for example, in the Japanese Laid-Open Patent Publication No. H 7-74584, No. H 6-29779, No. H 9-205343, and No. H 11-251871. These publications disclose the SAW filter that decreases the current value per unit area by increasing the cross length and number of cross couples of the SAW resonator, so as to cope with a higher power.
However, when the SAW filter capable of handling a high power is employed for the SAW branching filter, and has a high power inputted thereto, there can be created a deterioration of the branching filter characteristics. There are inevitably large variations of the antenna impedance in the mobile communication terminals. Accordingly, there arises a problem of characteristic deterioration by the variations of the antenna impedance. That is, in either case of the antenna terminal and the receiver terminal being terminated with 50Ω, and the antenna terminal and the receiver terminal being opened, it is preferred that there do not occur characteristic variations. However, when the antenna terminal and the receiver terminal are opened, there is a generally known problem of characteristic deterioration of the Tx filter or a signal disconnection by series arm destruction.
In the mobile communication terminals such as portable telephones, the SAW branching filter is requested to have the function that uses one antenna for both transmission and reception, and further to have the following function. That is, the filter is requested to satisfy the required characteristics in the following cases:    (1) when the antenna is normally operated, namely, when the antenna impedance is 50Ω, and    (2) when the antenna is opened, namely, when the antenna impedance is infinite.
In the aspect of change in the foregoing two cases, the impedance varies sharply. During the transmission having a power applied to the SAW branching filter, there is a possibility that can produce a characteristic variation or a destruction of the SAW branching filter.